1. Field of the Invention
The present invention relates to an electrolyte solution for a magnesium rechargeable battery that is electrochemically stable in a high-voltage region and has an improved ionic conductivity, and a method for preparing the electrolyte.
2. Description of the Related Art
It is well known that the conventional lithium batteries employing lithium metal suffer from the many parasitic reactions of electrolyte systems towards reactive lithium anode, which poses several critical concerns in terms of their safety. Furthermore, lithium is known as an expensive element since it is relatively rarely found as a natural resources. Particularly, with the recent increasing demand for medium- and large-scale applications such as electric vehicles and ESS, safety and cost concerns of rechargeable lithium batteries become one of the dominating factors that should be taken into consideration and are considered to be major obstacles for the medium-/large-scale application.
In attempts to solve such problems, magnesium rechargeable batteries using magnesium metal as an electrode active material have recently been proposed as alternatives to rechargeable lithium batteries. Magnesium rechargeable batteries work by the migration of electrons during intercalation/de-intercalation of magnesium ions from a magnesium plate as an electrode, specifically an anode, into/from a cathode active material. Magnesium has a theoretical capacity similar to that of lithium and is environmentally friendly. Magnesium is far less expensive than lithium and is superior in terms of battery safety to lithium. Due to these advantages, magnesium rechargeable batteries have received considerable attention as a potential replacement for the rechargeable lithium batteries.
Previous research on the development of electrolyte system for the magnesium rechargeable batteries has focused on Grignard solutions (alkyl magnesium halide, RMgX, R=alkyl, X=halide) that shows a reversible Mg deposition and dissolution behavior on the electrode. However, Grignard solutions have low ionic conductivities which causes low charge/discharge rates of batteries, limiting the battery performance. Thus, fundamental improvements are needed to develop more competitive magnesium rechargeable batteries and magnesium hybrid batteries than existing ones.
Magnesium metal is a promising anode material for batteries due to its high energy densities per unit mass and volume (2205 Ah/kg, 3833 Ah/L, respectively). Particularly, magnesium is an abundant natural resource and is easy to handle. In addition, the use of magnesium as an anode material prevents the formation of dendrites on the electrode surface during charge and discharge. For these reasons, magnesium batteries are superior in safety and price competitiveness. In this aspect, magnesium batteries have received a great deal of attention as medium- and large-size battery systems for electrical energy storage and electric vehicles whose market is expected to expand in the near future.
The first serious study on the magnesium rechargeable batteries was first carried out by T. Gregory et. al. in 1990s although they belong to one of the battery systems with highest theoretical energy densities, only second to lithium batteries. However, for more than a decade after this report, there have been few reports on magnesium batteries. In the 2000's, D. Aurbach's group at Bar-Ilan University developed Chevrel-phase cathode active materials to ensure reversibility. Since then, magnesium batteries have again begun to attract much attention as promising alternatives to rechargeable lithium batteries for their ability to solve the safety and price problems of lithium batteries. However, the energy density of magnesium rechargeable batteries developed hitherto is half or less than that of lithium-ion batteries. Under these circumstances, there is an urgent need to develop new cathode active materials, electrolyte materials, and current collectors.
Particularly, there are many challenges related to the reversible deposition and dissolution of magnesium metal on the negative electrodes, the reversible insertion and de-insertion of Mg2+ ions into the cathode materials, and the diffusion of Mg2+ ions within the solid phase. A key solution to these challenges is to develop new electrolytes applicable to both cathode and anode.
Cathode active materials and electrolytes are two main research fields in the development of magnesium batteries. In the field of cathode active materials, various compounds, such as metal-sulfur compounds, organosulfur compounds, metal oxides, and metal silicate compounds, are being investigated to achieve high reversible capacity per unit weight and enhanced reversibility. However, the performance of these compounds is not yet satisfactory. In the current state of the art, the only cathode active materials such as Chevrel-phase Mo6S8 and Mo6Se8 show battery performance suitable for commercialization.
In the field of electrolytes, most studies have focused on Grignard solutions that are reversible with magnesium anodes. In recent years, some magnesium materials, including magnesium aluminate, have been reported to exhibit excellent performance characteristics. However, Grignard electrolytes capable of reversibly depositing and dissolving magnesium are very reactive with common cathode materials due to their high reducing power, making it impossible to practically apply to batteries. In contrast, the conventional electrolytes based on magnesium salt dissolved in organic solvent capable of reversibly intercalating/deintercalating Mg2+ ions into/from cathode materials form thick passivation films on the surface of magnesium anodes, impeding reversible deposition and dissolution of the metal.
U.S. Pat. No. 6,713,213 to Matsushita Electric Industrial Co., Ltd. suggests a non-aqueous magnesium rechargeable battery comprising a rechargeable positive electrode, a non-aqueous electrolyte, and a rechargeable negative electrode, wherein the non-aqueous electrolyte contains a halogen-containing organic magnesium compound represented by RMgX.
Japanese Patent Publication No. 2007-188709 to Sony suggests an electrochemical device having a first electrode, a second electrode, and an electrolyte wherein an active material of the second electrode forms magnesium ions as a result of oxidation and the electrolyte is a mixture of a Grignard solution represented by RMgX (where R is an alkyl or aryl group and X is a fluorine, chlorine or bromine) and an organometallic compound or a salt other than magnesium salts.
However, these electrolytes have poor electrochemical stability at high potential region and low ionic conductivities, and the battery and the device have low charge/discharge rates at high voltages, limiting their performance. Thus, further improvements are needed to develop competitive high voltage magnesium batteries and magnesium hybrid batteries in comparison with existing batteries.